Radial compressors for industrial use generally have a wide range of throughputs with excellent efficiencies under a variety of intake conditions. Radial compressors for use in the aeration of clarifier basins in waste water or sewage treatments are operated with a range of throughputs to supply the aeration devices in the clarifier basins with air. The air is generally blown into the clarifier basin from the bottom and the pressure which the air must be compressed for the delivery of a certain throughput in the aerating unit, is dependent upon the water level in the clarifier basin. During the course of the day the operating point may fluctuate because of changes in the intake temperature of the air. In addition, there are seasonal effects on the operation of the radial compressor set. As a result of these various influences, the operating point of the radial compressor in the operational characteristic diagram mapped on a graph plotted pressure versus the mass flow rate of the air may vary continuously or frequently. Furthermore, there are industrial applications of radial compressors in which the intake pressure undergoes significant fluctuations and these too can vary the operating points in such a diagram.
As a consequence, it is common to provide upstream of a radial compressor, an intake flow control and downstream of the radial compressor a discharge flow control each having a plurality of vanes and respective drives with variable output speeds or rpm. Adjustment of the discharge flow control can be effected when the compressor must cover a wide throughput range with a substantially constant isentropic compressor work. A regulation of the intake flow control can be effected when the compression ratio and/or the intake temperature significantly varies.
Neither the control of the intake flow control unit nor the control of the discharge flow control unit suffices alone to enable operation of the compressor with optimum efficiency.
When the regulation of the discharge flow control unit is combined with regulation of the intake flow control unit, there is practically an unlimited number of possible combinations for the angular positions of the vanes of the intake and discharge controllers and it is thus possible to establish a predetermined volume flow for the air to be compressed as well as a predetermined compressor work. There is, however, a problem with respect to setting an appropriate combination of vane positions in order to minimize the power consumption and thereby ensure the maximum operating efficiency of the compressor.
In a process described in the German patent document DE-A 195 06 790, attempts are made to optimize the efficiency of a radial compressor by measuring the intake temperature, the intake pressure, the discharge temperature, the discharge pressure and the throughput through the radial compressor whereby, using a calculation, the operating point can be associated with values for setting the angular positions of the vanes or blades of the intake control unit and the discharge control unit. The association is effected by means of an operational characteristic diagram stored in a computer and which contains a network of efficiency-optimizing characteristic line points. The characteristic line points are determined by empirical efforts in a series of tests on the original compressor or in models thereof. For each characteristic line and hence each of the points along such a line, a multiplicity of tests must be made in which the positions of the intake control unit and of the discharge control unit are systematically varied and the respective efficiencies of the machine are determined.
Those combinations which maximize the efficiency of the compressor are inputted as the optimum characteristic lines or characteristic line points in the operational characteristic diagram. The operational characteristic diagram and the characteristic lines must be newly determined for each radial compressor and hence the operation is expensive. Furthermore, the determination of the operational characteristic diagrams and performance characteristics requires expensive computer-based interpolation processes for associating the respective volume flows with the working points given by the compressor work and thus to establish the sort of parameter combinations. In practice, this earlier process is too expensive and therefore is not used.
From European patent document EP 0 761 981, an operation of a radial compressor with adjustable intake and discharge control units is described in which the adjustment of the intake control unit has primacy. When the adjustment of the intake control unit does not suffice to establish a stable operation of the radial compressor, the discharge control unit is additionally adjusted. Operation at an optimum efficiency over a wide regulation range is however not always ensured by this process.